Learning Center Library Contents

Weather Radar Echo Terms

Author: Christine Soucy and Michael Lenz

Date: March 2006

Introduction

When thunderstorm season begins this year, pilots will start hearing some very important
changes in the way Air Traffic Control (ATC) describes radar weather echoes to pilots.
Beginning in late spring 2006, pilots will hear ATC use four terms:

"Light"

"Moderate"

"Heavy," and

"Extreme"

to describe weather radar echoes. Each term represents a precipitation intensity level
paired with a dBZ range to help pilots interpret the severity of the flight conditions
present. (A dBZ is a measure of radar reflectivity in the form of a logarithmic power
ratio (in decibels or dB) with respect to radar reflectivity factor "Z.")
The four terms will be used universally in the National Airspace System (NAS) by
approach controllers and Air Route Traffic Control Center (ARTCC) and Automated Flight
Service Station (AFSS) specialists. The decision to standardize the terminology was easy
to make, because the ARTCC facilities and many of the terminal approach control facilities
now have digital radar display systems with processors that can better determine
the intensity (dBZ) of radar weather echoes and display that information to the controller.

What ATC Sees

Most of us are familiar with The Weather Channel and local news and weather broadcasts
that use the Doppler NEXRAD (next generation radar) WSR 88D weather radar. Some of you
may even use those broadcasts to supplement your flight planning and overall weather
awareness. However, there are significant differences with how weather information
is displayed on a controller's radarscope and the local news weather broadcast depictions.
NEXRAD is designed to detect and display weather, but ATC radar systems are designed
to detect and display aircraft. Because the NEXRAD color coding and 16 individualized
precipitation levels can provide excess clutter and possibly compromise the ability
of controllers to safely perform their duties, different systems for depicting weather
radar echoes needed to be developed for the ATC environment.

Air Route Traffic Control Centers

In air route traffic control centers, NEXRAD data is fed through the Weather And Radar
Processor (WARP) that organizes the 16 NEXRAD levels into four reflectivity (dBZ)
categories. Reflectivity returns of less than 30 dBZ are classified as "LIGHT"
and are filtered out of the center controller's display. The remaining three
categories correlate to bands of dBZ values to assist pilots in evaluating the
severity of flight conditions that might be associated with those precipitation
returns. Therefore, the wide range of color coding available to NEXRAD is not available
to the controller, and the ARTCC's WARP system does not display dBZ levels below 30.
Therefore, center controllers will not be able to report areas of "light"
weather radar echoes.

WARP/NEXRAD is a vast improvement over the Air Route Surveillance Radar (ARSR)
display of weather radar echoes that center controllers used exclusively prior to
the implementation of the NEXRAD type weather radars. The ARSR displays the echoes
to the controller by indicating "moderate" intensities with a slash mark
"/" and more intense areas with the letter "H."

Approach Control and Terminal Facilities

In the approach control world, neither NEXRAD nor WARP is available. Instead, radar
weather echoes are displayed by the Airport Surveillance Radar (ASR) systems using
Common Automated Radar Terminal System (Common ARTS) or Standard Terminal Automation
Replacement System (STARS) digital processors. The digitized ASR 9 and 11 systems (and
some ASR 8 systems that have been digitized) paired with a weather processor, display
the four weather radar echo intensity categories to the controller.

Terminal facilities can and do display "light"(less than 30 dBZ) areas
of precipitation.

...But Keep in Mind...

Of course, there are no absolutes. In the universe of terminal radars, the NAS
still has a few non-digital ASR systems. While these systems do a good job of
displaying weather radar echoes, they lack processors that can discern the intensity
of the echoes. These facilities will not be able to use the terms,
"light," "moderate," "heavy," or "extreme."
Controllers who work from these displays will be able to tell pilots the position
of weather radar echoes but will state, "intensity unknown" because their
system does not indicate what dBZ level of reflectivity is present.

Important Things to Know

In the world of ATC, weather radar echoes are all referred to as
"precipitation" even though, technically, it is possible the echo could
be associated with birds, volcanic ash, etc., or precipitation that is not reaching
the Earth's surface (virga). Controllers will tell pilots the location of significant
areas of "precipitation" when it appears that it may affect the aircraft's
flight path. They will also provide assistance in the form of course deviations when
requested by the pilot.

Rainfall rates (i.e., inches/hour) as they relate to intensity (dBZ) have not been
correlated with the ATC displays. Therefore, the terms (light/moderate/heavy/extreme)
cannot be equated/correlated to rates of rainfall per se at this time.
Generally, however, the more intense the echo, the more likely there is to be greater
intensities of precipitation; and when conditions are favorable for convective activity,
turbulence and other weather hazards should be expected. As the intensity of precipitation
increases, so too, does the likelihood of more severe weather conditions. Pilots should
also remember that turbulence can be present in areas where ATC does not display
precipitation at all. Therefore, pilots should always exercise care when transiting
areas of known or suspected convective activity.

Important Things to Do

Pilots of light general aviation aircraft should even approach areas of
"light" precipitation with caution. A rapidly growing thunderstorm can
increase at a rate of 6,000 feet per minute! Think of the time-lapse photographs and
weather radar loops showing building thunderstorms. "Light" precipitation
could grow to "moderate" and "heavy" levels within a very short
period of time, given the right conditions.

The following tips are offered to assist pilots in navigating stormy skies safely.

Request course deviations early. Don't wait until the last moment.

Ask for information updates as needed. The ARTCC WARP/NEXRAD updates every one to
six minutes. Terminal (ASR based) systems show near "real time" echoes.

Make sure the controller understands what services you want.

Maintain situational awareness concerning your position and the weather areas you wish
to avoid.

Include the information that you are on a heading assigned/approved by ATC for weather
avoidance, when you report onto the next controller's frequency.

Verify what additional services ATC is providing to you. Is it what you need?

Pipe Up with a PIREP

Pilot reports (PIREP) of flight conditions are an invaluable source of information for
other pilots and controllers as well. PIREPs should include reports of turbulence, icing,
cloud tops and bases, intensity of rain, presence of hail, sleet, etc. A PIREP is often
the only source of information regarding actual flight conditions a pilot may encounter.
Do your part for flight safety and pipe up with a PIREP!

For those who have never submitted a PIREP, the Aeronautical Information
Manual (AIM) explains how to submit one, the uses of a PIREP,
and the format a pilot should use in reporting information. AIM paragraph 7-1-21,
Pilot Weather Reports, is the reference. Table 7-1-6, PIREP Element
Code Chart, explains the reporting format with the elements explained.

About the Authors

Christine Soucy is with FAA's Office of Accident Investigation, Accident Coordination
Branch. Michael Lenz is a Program Analyst in the Flight Standards Service's General
Aviation and Commercial Division.

Note: Please download the PDF version to see the graphics associated with
this article.